Lift tables or lift elevator mechanisms are used in a number of different industries for presenting a stack of sheet materials to a work station or for receiving sheet materials from a work station and stacking them for subsequent removal to a storage or shipping facility. These lifts or elevators are used to raise a stack of sheet materials, so that the uppermost sheet in the stack always is presented to the work station at the same height. Similarily, when sheets are removed from a work station, the lift or elevator mechanism is lowered in a step-by-step manner; so that each new sheet which is transferred from the work station to the lift mechanism is transferred onto the top of the stack of previously-transferred sheets level with the top of the stack. Lift mechanisms are particularly suited for feeding large sheets of particle board, plywood, wallboard or the like onto a work station where the boards are cut, shaped, coated or otherwise acted upon prior to removal from the work station to another lift mechanism where the finished sheets of material are stacked.
In a typical factory environment a large stack of plywood sheets or the like are usually placed on a scissors lift table. The height of the table then is hydraulically or electrically adjusted by an operator and, as sheets are removed from the table onto the work station (or are removed from the work station onto a similar lift table at the receiving side), the operator usually uses a foot lever to raise (or lower) the lift mechanism to put the next sheet in the stack at the level from which it can be readily moved off the stack to the work station (or from the work station onto the receiving stack). These devices usually require the presence of an operator at all times and they are relatively cumbersome and expensive. In addition, the lifting mechanism is located directly beneath the lifting surface of the table and occupies a substantial amount of vertical space, even when the table is in its lowermost position. Consequently, there is a significant amount of space between the lowermost position of the lifting surface of the table and the floor which, in turn, limits the amount of material which can be stacked on the table for supply to or removal from a work station located adjacent the table. Patents which are typical of such scissors lift mechanisms are the patents to Kohler, U.S. Pat. No. 3,067,885, and Herman, et al., U.S. Pat. No. 2,960,805.
A slightly different approach, but one which also is subject to the disadvantages of the wasted space beneath the lifting surface present in the scissors lift devices, is described in the patent to Sparks, U.S. Pat. No. 2,595,015. This patent discloses a hydraulic lift system in which the hydraulic lift is located directly beneath the lift table. Because of this location, it is necessary to mount the hydraulic cylinder beneath the floor under the table. This is an expensive installation which, obviously, is inflexible once it is installed since it must remain in place in the position of the original installation.
Lift mechanisms or lift elevators which are capable of more efficient utilization of the vertical space between the floor and the lift surface are disclosed in the patents to Bronson, U.S. Pat. No. 3,123,024; Harred, U.S. Pat. No. 2,381,743; Pabich, U.S. Pat. No. 2,467,493; Shorthouse, U.S. Pat. No. 4,172,686; Larsson, U.S. Pat. No. 3,674,240; Charbonnet, U.S. Pat. No. 4,373,846; and Wilson, U.S. Pat. No. 3,490,614. The Bronson patent discloses a spring-operated leveling device of the type used in restaurants for presenting racks of dishes to a uniform work height (such as for a dishwasher or the like). The lift arms of the elevator mechanism are supported by a pair of spaced rollers bearing against opposite sides of a guide channel formed by a pair of upright members 17 of the mechanism. This roller arrangement permits the load to be held on cantilevered arms or a cantilevered platform while it moves vertically on the rollers confined within the guide channel.
The rest of the patents mentioned in the preceding paragraph are directed to elevator-type mechanisms for transferring sheet material from stacks to a uniform work height. Several of these patents disclose the use of a sensing switch to locate the position of the top sheet in the stack. All of these patents disclose the use of either hydraulic lifts or other types of mechanical mechanisms, such as chains or cables, to vertically move a pair of spaced-apart lift arms or a lower lift panel underneath the stack of articles to be presented to the work station.
For those patents of this group which disclose cantilevered spaced-apart arms or a cantilevered platform for lifting the load, the various mechanisms are capable of the necessary step-by-step incremental positioning of the load as articles are removed through it under the control of the "top article" sensing switch. None of these patents, however, provide any means for compensating for deflection or bending of the cantilevered lift arms or platform under the weight of the load. It is possible, for example, when plywood sheets constitute the load, for the load to be extremely heavy and to cause a relatively large deflection of the ends of the cantilevered lifting arms when they are subjected to a full load or nearly full load. Consequently, compensation in the form of additional mechanisms or guide surfaces or the like must be provided on either the work station or the lift mechanism or both to adjust the position of the top sheet being transferred to the work station, depending on how many sheets constitute the load on the lift mechanism at any given time. As the load is removed sheet-by-sheet from the lift mechanism, it raises; and, as the weight diminishes, the deflection or bending of the lift arms also diminishes. Accordingly, it has been difficult to provide close tolerances whenever heavy loads are being transferred from the lift table. In addition, it has been difficult to provide accurate automatic sensing of the top sheet relative to the work table, since the orientation of the top sheet varies as the load varies, unless extremely heavy-duty lift arms and corresponding heavy-duty hydraulic systems are employed simply to overcome the potential for a bending deflection of the lift arms by brute force.
Another shortcoming of the patents which have been discussed above is that many of them are incapable of being loaded and unloaded by the fork-lift trucks used extensively throughout industry today. Those which can be loaded by means of a fork-lift truck do not present a clear area at ground level; so that the lift arms of the fork-lift truck cannot deposit stacks of materials onto the lift mechanism unless the materials are placed on a spacer or pallet.
Accordingly, it is desirable to provide a lift mechanism or lift table which is not subject to the disadvantages of the prior art discussed above. More specifically, it is desirable to provide a lift table which is capable of presenting the top sheet of the load at the same orientation (preferably horizontal) irrespective of the number of sheets or articles which are stacked on the lift mechanism at any given time. It also is desirable to provide a lift table or lift mechanism which readily can be loaded and unloaded by a standard fork-lift truck.